Field of the Invention
The present invention is broadly concerned with thin-film, resistive-based sensors.
Description of the Prior Art
Sensors able to detect environmental changes are in demand for a number of applications. Detection of changes in temperature, pressure, or various analytes in contact with the sensor (including moisture or gases in the air, etc.) is desirable for applications such as indoor and outdoor climate detection and control, process control, biometrics, medical uses, and much more. However, the usefulness of these sensors for various applications is limited by their ability not only to accurately and precisely measure stimuli, but their ability to detect and measure those stimuli in both the short and long term. Response time and settling time are very important specifications when trying to measure humidity for applications where immediate detection is necessary, such as breathing sensors. The response time of a sensor is the time it takes for a sensor to respond from no load to a step change in load. Settling time is the time it takes for a sensor to reach a stable output once it is turned on. Sensors must also be able to maintain long-term stability and measurement, and should demonstrate low hysteresis.
An environmental sensor can detect a physical change in the atmosphere, such as temperature, humidity, gas, or airflow. Through a sensing element, the sensor converts the variation through a transduction element into electrical signals that may be transmitted and measured. In many practices, the electronic resistance of the “active” layer is measured during a change in the sensor environment. If this measurement is single valued, stable, and reproducible, it can be calibrated and used as a signal indicative of the environment surrounding the sensor.
There are different approaches to the architecture and composition of environmental sensors. These approaches seek to increase one or more characteristics of efficient sensing capabilities: stability, sensitivity, low hysteresis, reliability, and/or accuracy. The most basic printed and/or thin film resistive-based sensors usually include a thin electronically “active” sensing layer printed on a dielectric substrate with a passivation or protection overlayer printed on the top surface.
Many prior art direct-current, resistive-based sensors have been plagued with poor performance as a result of very high hysteresis, low stability, and low accuracy. As these devices often utilize polymer-based materials or unpredictable materials, the problems of high hysteresis and low stability and accuracy are often attributed to the material choices used for the sensing element in the thin-film structure.
Other sensor technologies that are capacitance, inductive, optical, and physical-based and utilize established materials do not generally suffer from this type of poor performance. However, even though more reliable, these sensor technologies are slow.